Static random access memory device having reduced leakage current during active mode and a method of operating thereof

ABSTRACT

An Static Random Access Memory (SRAM) device and a method of operating the same. In one embodiment, the SRAM device includes: (1) an SRAM array coupled to row peripheral circuitry by a word line and coupled to column peripheral circuitry by bit lines and (2) an array low voltage control circuitry that provides an enhanced low operating voltage V ESS  to the SRAM array during at least a portion of an active mode thereof.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to memory devices and,more specifically, to a Static Random Access Memory (SRAM) device havingreduced leakage current during active mode and a method of operatingthereof.

BACKGROUND OF THE INVENTION

Memory devices are well known in the art and used in, among otherthings, virtually all microprocessor and digital signal processor (DSP)applications. One type of memory favored for many applications is StaticRandom Access Memory (SRAM). SRAM devices are fast and easy to userelative to many other types of memory devices. In addition, SRAMdevices using metal-oxide-semiconductor (MOS) technology exhibitrelatively low standby power and do not require a refresh cycle tomaintain stored information. These attributes make SRAM devicesparticularly desirable for battery-powered equipment, such as laptopcomputers and personal digital assistants.

Miniaturization of SRAM devices is another attribute that may make SRAMdevices desirable for such battery-powered equipment. Desiredminiaturization, however, may include undesirable operating problems forthe smaller SRAM devices. For example, current leakage may increase asthe size of SRAM devices decreases. Current leakage may provide aproblem during a sleep mode, a standby mode and even during an activemode. In fact, current leakage, which may be represented by I_(DDQ), hasbecome a greater concern during the active mode as SRAM devices continueto decrease in size.

Some options to reduce current leakage during the active mode alreadyexists. In one such option, for example, the high operating voltage,V_(DD), supplied to the SRAM device may be reduced to lower the voltageacross the SRAM array. A reduced high operating voltage V_(DD), however,can also reduce static noise margin (SNM) and write trip voltage(so-called “V_(trip)”) below acceptable levels. Alternatively, a lowoperating voltage, V_(SS), supplied to the SRAM device can be raisedwhile in a standby mode and lowered when in the active mode. Thisalternative, unfortunately, requires more switching power and does notsupport the write trip voltage V_(trip).

A high V_(trip) and SNM are desired cell characteristics of SRAMdevices. A high SNM is desired for circuit stability. If SNM is too low,READ operations may be disrupted. A high V_(trip) is desired foradequate data write speed. If V_(trip) is too low, WRITE operations maybe disrupted. Unfortunately, the requirements for an acceptable SNM andwrite trip voltage V_(trip) may limit the tolerances for an acceptableSRAM yield during manufacturing since increasing one typically decreasesthe other.

A typical six-transistor SRAM memory cell (the basic unit of a SRAMdevice) consists of two p-channel “pull-up” transistors, two n-channel“pull-down” transistors and two access transistors, which are typicallyn-channel transistors. The strength of the p-doped and n-doped channelsof the transistors affects the performance of the SRAM memory cell as awhole. For example, a strong n-channel can cause SNM to be unsuitablylow, particularly when accompanied by a weak p-channel. One might betempted to weaken the n-channel and/or strengthen the p-channel toachieve a satisfactory SNM. However, a weak n-channel can cause V_(trip)to be unsuitably low, particularly when accompanied by a strongp-channel.

Thus, existing SRAM devices are challenged by the competing andcontradicting objectives of providing a weak n-channel (and/or a strongp-channel) to achieve an acceptable SNM and providing a strong n-channel(and/or a weak p-channel) to achieve an acceptable V_(trip). Moreover,this trade-off between SNM and V_(trip) (and, thus, between reliableREAD and WRITE operations) becomes increasingly constrained withcontinued miniaturization and lower operating voltages since theseamplify the effect of normal manufacturing variations.

Even though both SNM and V_(trip) are degraded when the high operatingvoltage V_(DD) is reduced, a reduction of the high operating voltageV_(DD) is often desirable for low power operation and for some testconditions. Though both SNM and V_(trip) are degraded, the degradationof V_(trip) is particularly strong with a reduction in the highoperating voltage V_(DD). Thus V_(trip) tends to limit the lower boundof the high operating voltage V_(DD) for operation. A solution forreducing current leakage during an active mode, therefore, needs toprovide a minimum adverse affect on SNM and V_(trip).

Accordingly, what is needed in the art is an improved SRAM device havingreduced current leakage during an active mode. Additionally, theimproved SRAM device with reduced current leakage needs a strong SNM andwrite trip voltage V_(trip).

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides an SRAM device and a method of operatingthereof. In one embodiment, the SRAM device includes (1) an SRAM arraycoupled to row peripheral circuitry by a word line and coupled to columnperipheral circuitry by bit lines and (2) an array low voltage controlcircuitry that provides an enhanced low operating voltage V_(ESS) to theSRAM array during at least a portion of an active mode thereof.

The present invention, therefore, may provide a greater power savingsand a better SNM and write trip voltage V_(trip) trade-off than existingSRAM devices allowing a wider process margin. Providing the enhanced lowoperating voltage V_(ESS) reduces current leakage I_(DDQ) by loweringvoltage across the memory cell and raising the threshold voltage V_(tn)due to a back bias effect since the enhanced low operating voltageV_(ESS) is greater than a substrate voltage (typically the low operatingvoltage V_(SS)). The SNM may be improved by the higher V_(tn) from theback bias effect that adequately compensates for the lowering of the SNMby a reduced cell voltage. With an improved SNM, cells may suffer aminimum of upset when the associated word line voltage is raised foraccess, as in a READ, or for non-addressed columns in an accessed rowfor WRITE. Similarly, the improved SNM may allow WRITE operationswithout upsetting cells in unaddressed columns.

A down side to the enhanced low operating voltage V_(ESS) may be areduction in the read current I_(read). I_(read), however, may berestored by providing the enhanced low operating voltage V_(ESS) at alower value for addressed columns during a READ operation. The presentinvention may also provide the enhanced low operating voltage V_(ESS) ata lower value for READ operations than for WRITE operations.Additionally, the enhanced low operating voltage V_(ESS) may be set tocompensate for process variations by, for example, providing theenhanced low operating voltage V_(ESS) at a higher value for processcorners where failures of the SNM and the write trip voltage V_(trip)may occur. The enhanced low operating voltage V_(ESS) may also beprovided at a higher value when the write trip voltage V_(trip) is at aworst case. Furthermore, the enhanced low operating voltage V_(ESS) maybe provided at a higher value for low voltage operations, such as,during testing.

Thus, the enhanced low operating voltage V_(ESS) may be defined atdistinct values based on certain factors, such as, transistor parametersor modes of operations. For example, one enhanced low operating voltageV_(ESS) value may be established for a test operation. Additionally,different enhanced low operating voltage V_(ESS) values may beestablished for a READ operation or a WRITE operation. In someembodiments, a single enhanced low operating voltage V_(ESS) value maybe established for more than one factor. Therefore, a single enhancedlow operating voltage V_(ESS) value may be provided for a TEST, WRITE orREAD operation.

In another aspect, the present invention provides a method of operatingan SRAM device including (1) employing in an integrated circuit an SRAMarray coupled to row peripheral circuitry by a word line and coupled tocolumn peripheral circuitry by bit lines and (2) providing an enhancedlow operating voltage V_(ESS) to the SRAM array during at least aportion of an active mode.

The foregoing has outlined preferred and alternative features of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following detailed description taken in conjunction withthe accompanying FIGUREs. It is emphasized that various features may notbe drawn to scale. In fact, the dimensions of various features may bearbitrarily increased or reduced for clarity of discussion. Reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a circuit diagram of an embodiment of an SRAM deviceconstructed according to the principles of the present invention; and

FIG. 2 illustrates an embodiment of a method of operating an SRAM devicecarried out according to the principles of the present invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a circuit diagram of anembodiment of an SRAM device, generally designated 100, constructedaccording to the principles of the present invention. The SRAM device100 includes an SRAM array 110, row peripheral circuitry 120, columnperipheral circuitry 130 and an array low voltage control circuitry 140.Typically, the SRAM array 110 includes multiple memory cells organizedin a matrix of columns and rows with corresponding multiple word linesand bit lines. For example, the SRAM array 110 may have 256 columns and256 rows of memory cells. For ease of discussion, however, only a singlerow with a first, second and third column 112, 114, 116, of the SRAMarray 110 are illustrated. Furthermore, only a single word line, bitline pair, row peripheral circuitry 120 and column peripheral circuitry130 associated with the first column 112 are illustrated and discussed.One skilled in the art, however, will understand that the illustratedand discussed word line, bit line pair, row peripheral circuitry 120 andcolumn peripheral circuitry 130 associated with the first column 112 mayfunctionally represent similar circuitry associated with the second andthird columns 114, 116, as well as additional columns and rows notillustrated.

The SRAM device 100 may be a memory component for an associatedmicroprocessor, digital signal processor (DSP), application-specificintegrated circuit (ASIC) or larger electronic apparatus. In someembodiments, more than one SRAM device 100 may comprise the memorycomponent. An Integrated Circuit (IC) associated with the SRAM device100 may supply a high operating voltage V_(DD) and a low operatingvoltage V_(SS), thereto. The high operating voltage V_(DD) and the lowoperating voltage V_(SS) may be chip supply voltages. Typically, signalpaths and a data interface of the associated apparatus may be coupled tothe SRAM device 100 to send address information and retrieve/send datafor reading/writing the data to specific memory cells of the SRAM array110. One skilled in the art will understand coupling of the SRAM device100 to the associated apparatus.

Each of the first, second and third columns 112, 114, 116, may includememory cells in multiple rows having a unique address for writing andreading the data. Each of the memory cells may employ MOSFETs and aflip-flop to store the data as a one or a zero. Reading and writing ofthe data in the first column 112 may be controlled by the row peripheralcircuitry 120 and the column peripheral circuitry 130.

The row peripheral circuitry 120 may control activating a word lineassociated with one of the rows. The row peripheral circuitry 120 mayinclude, for example, a row pre-decoder, a row decoder, a word linedriver and a keeper. The word line driver may activate the word line forreading or writing based on an address signal received via a row signalpath and decoded by the row pre-decoder and the row decoder.

The column peripheral circuitry 130 controls selecting the columns ofthe SRAM array 110 for reading and writing. The column peripheralcircuitry 130 may include, for example, pre-charge circuitry, writtencircuitry, a column multiplexer and a sense amplifier. Additionally, thecolumn peripheral circuitry may include address decoders for determiningmemory cell locations within the SRAM array 110 and control circuitryfor determining between writing or reading the data. The pre-chargecircuitry, the write circuitry, the column multiplexer and the senseamplifier may facilitate reading and writing data to the correct columnaddress that has been decoded. As with the row peripheral circuitry 120,the column peripheral circuitry 130 may also include additionalcomponents that facilitate writing and reading the data that are notillustrated or discussed herein.

The array low voltage control circuitry 140 may be configured to providean enhanced low operating voltage, V_(ESS), to the SRAM array 110 duringat least a portion of an active mode of the SRAM array 110. Of course,the enhanced low operating voltage V_(ESS) may be provided during all ofthe active mode. The array low voltage control circuitry 140 may employactive components to provide the enhanced low operating voltage V_(ESS).For example, the array low voltage control circuitry 140 may include adiode-bridged footer to raise the low operating voltage V_(SS) andprovide the enhanced low operating voltage V_(ESS). A footer may be atransistor positioned between the low operating voltage V_(SS) and theSRAM device 100. Typically, the footer may be an n-channel MOSFET. Thearray low voltage control circuitry may be an element to provide avoltage drop between the array and a low operating voltage supply bus.For example, the array low voltage control circuitry may be a turned ontransistor where the width of the turned on transistor determines thevoltage drop. The array low voltage control circuitry 140 may alsoemploy fuses to apply the enhanced low operating voltage V_(ESS) or toselect a particular value for the enhanced low operating voltageV_(ESS). The fuses may be employed to select a particular enhanced lowoperating voltage V_(ESS) value for a mode of operation, a value of thehigh operating voltage V_(DD) or transistor parameters. Of course, thefuses may also be employed to select other enhanced low operatingvoltage V_(ESS) values based on various factors.

Additionally, the array low voltage control circuitry 140 may employother components such as ROM or a voltage regulator to provide theenhanced low operating voltage V_(ESS). The array low voltage controlcircuitry 140 may also include logic circuitry to selectively providethe enhanced low operating voltage V_(ESS) to the SRAM array 110 basedon, for example, a WRITE operation, a READ operation, a test mode or aprocess corner. The logic circuitry may employ fuses or transistors toprovide such selectivity. The logic circuitry may be an associatedmicroprocessor that instructs selectivity of the array low voltagecontrol circuitry 140.

In some embodiments, the array low voltage supply 140 may provide theenhanced low operating voltage V_(ESS) only during a WRITE operation. Inother embodiments, the array low voltage control circuitry 140 mayprovide the enhanced low operating voltage V_(ESS) at a lower valueduring a READ operation than during a WRITE operation. The array lowvoltage control circuitry 140 may only provide the lower value for anaddressed column of the SRAM array 110. For instance, the row peripheralcircuitry 120 and the column peripheral circuitry 130 may indicate aREAD operation at the selected word line of the first column 112.Accordingly, the array low voltage control circuitry 140 may provide theenhanced low operating voltage V_(ESS) lower value to the first column112 to improve the read current during the READ operation. In someembodiments, the enhanced low operating voltage V_(ESS) lower value maybe provided to an appropriate block of the SRAM array 110 instead of acolumn.

In other embodiments, the array low voltage supply 140 may provide theenhanced low operating voltage V_(ESS) during all of the active mode.The array low voltage control circuitry 140 may also provide enhancedlow operating voltage V_(ESS) during all modes. For example, the arraylow voltage control circuitry 140 may provide the enhanced low operatingvoltage V_(ESS) for standby and sleep modes, too.

The enhanced low operating voltage V_(ESS) may be provided based oncharacteristics of the SRAM device 100 transistors. For example, thearray low voltage control circuitry 140 may provide the enhanced lowoperating voltage V_(ESS) based on a process corner. The array lowvoltage control circuitry 140 may provide the enhanced low operatingvoltage V_(ESS) at a higher value when based on a strong n processcorner. In other embodiments, the array low voltage control circuitry140 may provide the enhanced low operating voltage V_(ESS) based onfactors such as a mode of operation or a value of a high operatingvoltage V_(DD).

Turning now to FIG. 2, illustrated is an embodiment of a method ofoperating an SRAM device, generally designated 200, carried outaccording to the principals of the present invention. The SRAM devicehas a SRAM array that may vary in size. For example, the SRAM array mayhave 256 columns and 256 rows of memory cells. The SRAM device may beemployed in an Integrated Circuit (IC) that can include more than oneSRAM array. Typically, the SRAM array is provided a low and highoperating voltage, V_(SS), V_(DD), respectively. The low and highoperating voltages V_(SS), V_(DD), may be chip supply voltages. Themethod 200 begins with a desire to operate the SRAM device in a step205.

Next, an enhanced low operating voltage V_(ESS) is established in a step210. The enhanced low operating voltage V_(ESS) may be established basedon general characteristics of the SRAM device transistors. In someembodiments, an established enhanced low operating voltage V_(ESS) maybe altered based on a process corner. For example, an establishedenhanced low operating voltage V_(ESS) may be provided at a higher valuewhen the process corner is a strong n corner. The established enhancedlow operating voltage V_(ESS) may also be altered based on the highoperating voltage V_(DD) or temperature of the SRAM device. In someembodiments, however, the enhanced low operating voltage V_(ESS) may beindependent of the high operating voltage V_(DD) or temperature. Thus,the value of the enhanced low operating voltage V_(ESS) may vary basedon, for example, the process corner, the high operating voltage V_(DD)or temperature.

If the transistors do have a strong n process corner, then the enhancedlow operating voltage V_(ESS) may be provided at a higher value comparedto transistors that are not at a strong n process corner. In otherembodiments, the enhanced low operating voltage V_(ESS) may be providedat a higher value based on other transistor characteristics, such as, aweak p characteristic of the SRAM array transistors. For example, theenhanced low operating voltage V_(ESS) may be provided at about 0.1volts higher based on transistor characteristics. In some embodiments,the enhanced low operating voltage V_(ESS) may be provided at an evenhigher value if a lower value of the enhanced low operating voltageV_(ESS) is provided to addressed columns during a READ operation.

Different voltage values, therefore, of the enhanced low operatingvoltage V_(ESS) may be established for various operations. For example,one enhanced low operating voltage V_(ESS) value may be established fora test operation. Additionally, different enhanced low operating voltageV_(ESS) values may be established for a READ operation or a WRITEoperation. In some embodiments, a single enhanced low operating voltageV_(ESS) value may be employed for more than one type of operation.

After establishing the enhanced low operating voltage V_(ESS), adetermination is made if the SRAM device is in an active mode in a firstdecisional step 220. The SRAM device may be in the active mode during aWRITE or READ operation. Logic circuitry, such as a microprocessor,associated with the IC may determine when a WRITE or READ operationoccurs. Additionally, column and row peripheral circuitry may indicate aREAD or WRITE operation.

If the SRAM device is not in an active mode, then a non-active bias isapplied to the SRAM array in a step 225. If not in the active mode, theSRAM device may be in a standby mode or in a sleep mode. The non-activebias may be a low power bias designed to lower the high operatingvoltage V_(DD) or raise the low operating voltage V_(SS) to assist inretaining data during a non-active state. Optionally, the enhanced lowoperating voltage V_(ESS) may be applied to the SRAM array during thenon-active state. In some embodiments, an array low voltage controlcircuitry may provide the non-active bias or the enhanced low operatingvoltage V_(ESS). After applying the non-active bias, the method 200continues to a step 270.

Returning now to the first decisional step 220, if the SRAM device is inthe active mode, a determination is made if the SRAM device is in a testmode in a second decisional step 230. The logic circuitry associatedwith the IC may determine if the SRAM device is in the test mode. If theSRAM device is in the test mode, then the enhanced low operating voltageV_(ESS) test mode value is provided to the SRAM device in a step 235.The test mode value of the enhanced low operating voltage V_(ESS) may beprovided by the array low voltage control circuitry. In someembodiments, the enhanced low operating voltage V_(ESS) test mode valuemay be equivalent to the established enhanced low operating voltageV_(ESS). After providing the enhanced low operating voltage V_(ESS) testmode value, the method 200 continues to step 270.

If the SRAM device is not in the test mode, then a determination is madeif the SRAM device is in a READ mode in a third decisional step 240.Associated logic circuitry may determine if the SRAM device isperforming a READ operation. If in the READ mode, an enhanced lowoperating voltage V_(ESS) READ mode value is provided to the SRAM arrayin a step 245. The READ mode value may be provided by the array lowvoltage control circuitry. In some embodiments, the READ mode value mayonly be provided to a portion of the SRAM array. For example, anaddressed column of the SRAM array may be the only portion of the SRAMarray in which the READ mode is provided. In other embodiments, a blockof the SRAM array may be provided the READ mode value during a READoperation. The enhanced low operating voltage V_(ESS) READ mode valuemay be about the value of the low operating voltage V_(SS). Afterproviding the enhanced low operating voltage V_(ESS) READ mode value,the method 200 continues to step 270.

If the SRAM device is not in the READ mode, then a determination is madeif the SRAM device is in a WRITE mode in a fourth decisional step 250.Associated logic circuitry may determine if the SRAM device isperforming a WRITE operation. If in the WRITE mode, an enhanced lowoperating voltage V_(ESS) WRITE mode value is provided to the SRAM arrayin a step 255. The WRITE mode value may be provided by the array lowvoltage control circuitry. After providing the enhanced low operatingvoltage V_(ESS) WRITE mode value, the method 200 continues to step 270.

If the SRAM device is not in the WRITE mode, then the enhanced lowoperating voltage V_(ESS) is provided to the SRAM array in a step 260.The enhanced low operating voltage V_(ESS) may be provided by an arraylow voltage control circuitry. The enhanced low operating voltageV_(ESS) may provide a lower voltage across the SRAM array during theactive mode to reduce leakage current while maintaining an adequate SNMand V_(trip). The enhanced low operating voltage V_(ESS) may be providedby increasing the low operating voltage V_(SS) employing the array lowvoltage control circuitry. The enhanced low operating voltage V_(ESS)may be provided by employing active components such as a diode-bridgedfooter. Of course one skilled in the art will also understand that theenhanced low operating voltage V_(ESS) may be provided by employingother active components or passive components. For example, the enhancedlow operating voltage V_(ESS) may be provided by employing a resistor, atransistor, a diode, a low drop-out regulator or a combination thereof.In some embodiments, the enhanced low operating voltage V_(ESS) may beat about 0.2 volts. In other embodiments, the enhanced low operatingvoltage V_(ESS) may be at about the low operating voltage V_(SS).

After providing the enhanced low operating voltage V_(ESS), adetermination is made to continue operating the SRAM device in the fifthdecisional step 270. If operating the SRAM device continues, then themethod proceeds to step 210 and continues as described above. Oneskilled in the art will also understand that the high operating voltageV_(DD) may also be maintained to the SRAM array while operating. In someembodiments, the high operating voltage V_(DD) may be about 1.2 voltsand the low operating voltage V_(SS) may be about 0.0 volts. Ifoperating the SRAM device does not continue, then the method ends in astep 280.

While the methods disclosed herein have been described and shown withreference to particular steps performed in a particular order, it willbe understood that these steps may be combined, subdivided or reorderedto form an equivalent method without departing from the teachings of thepresent invention. Accordingly, unless specifically indicated herein,the order and/or the grouping of the steps are not limitations of thepresent invention.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form. For example, the SRAMarray may be at an enhanced low operating voltage V_(ESS) during allmodes, such as, the standby and sleep mode. Additionally, SRAM array maybe provided the enhanced low operating voltage V_(ESS) only during aWRITE operation. While in other embodiments, the enhanced low operatingvoltage V_(ESS) may be provided to the SRAM array during all of theactive mode.

1. An SRAM device, comprising: an SRAM array coupled to row peripheralcircuitry by a word line and coupled to column peripheral circuitry bybit lines; and an array low voltage control circuitry that provides anenhanced low operating voltage V_(ESS) to said SRAM array during atleast a portion of a READ operation or a WRITE operation thereof, saidenhanced low operating voltage V_(ESS) selectively adjusted to adistinct value according to device factors and having a higher valuethan a low operating voltage V_(ss), and wherein said array low voltagecontrol circuitry provides said enhanced low operating voltage V_(ESS)at a lower value during said READ operation than during said WRITEoperation.
 2. The SRAM device as recited in claim 1 wherein array lowvoltage control circuitry provides said enhanced low operating voltageV_(ESS) based on a factor selected from the group consisting of: aprocess corner, a transistor parameter, a mode of operation, and a valueof a high supply voltage.
 3. The SRAM device as recited in claim 1wherein said array low voltage control circuitry provides said enhancedlow operating voltage V_(ESS) at a higher value when based on a strong nprocess corner.
 4. The SRAM device as recited in claim 1 wherein saidarray low voltage control circuitry only provides said lower value foran addressed column of said SRAM array.
 5. The SRAM device as recited inclaim 1 wherein said array low voltage control circuitry employs anactive component to provide said enhanced low operating voltage V_(ESS).6. The SRAM device as recited in claim 1 wherein said array low voltagecontrol circuitry provides said enhanced low operating voltage V_(ESS)employing a component selected from the group consisting of: a diode, atransistor, a fuse, a ROM, a voltage regulator, and logic circuitry. 7.A method of operating an SRAM device, comprising: employing in anintegrated circuit an SRAM array coupled to row peripheral circuitry bya word line and coupled to column peripheral circuitry by bit lines; andproviding an enhanced low operating voltage V_(ESS) to said SRAM arrayduring at least a portion of an active mode, said enhanced low operatingvoltage V_(ESS) selectively adjusted to a distinct value according todevice factors and having a higher value than a low operating voltageV_(ss), and wherein said enhanced low operating voltage V_(ESS) isprovided at a lower value during a READ operation than during a WRITEoperation.
 8. The method as recited in claim 7 wherein said providingonly occurs during a WRITE operation.
 9. The method as recited in claim7 wherein said providing occurs during all of said active mode.
 10. Themethod as recited in claim 7 wherein said providing occurs during allmodes.
 11. The method as recited in claim 7 wherein said providing isbased on a factor selected from the group consisting of: a processcorner, a transistor parameter, a mode of operation, and a value of ahigh supply voltage.
 12. The method as recited in claim 7 wherein saidenhanced low operating voltage V_(ESS) is provided at a higher valuebased on a strong n process corner.
 13. The method as recited in claim 7wherein said lower value is only provided for an addressed column ofsaid SRAM array.
 14. The method as recited in claim 7 wherein saidproviding includes employing an active component to provide saidenhanced low operating voltage V_(ESS).
 15. The method as recited inclaim 7 wherein said providing includes employing a component selectedfrom the group consisting of: a diode, a transistor, a fuse, a ROM, avoltage regulator, and logic circuitry.